1. Field of the Invention
This invention relates to methods and apparatus for the characterization of a hydrocarbon mixture in accordance with its paraffinicity. More specifically, this invention relates to an improved method and apparatus for monitoring the Watson-Nelson characterization factor of hydrocarbon mixtures.
2. Description of the Prior Art
As was previously explained in my prior U.S. Pat. No. 3,557,609, the Watson-Nelson characterization factor, K, is a very useful tool for describing by one numerical quantity the relative paraffinicity of a petroleum crude oil or a fraction thereof. It was first described in the literature in 1933 in Ind. Eng. Chem., Vol. 25, page 880, by its originators, who observed an empirical relationship between specific gravity and average boiling point of different boiling range fractions from a given crude oil. This observation led to the development of the equation: ##EQU1## wherein: K = Watson-Nelson characterization factor;
T.sub.b = average boiling point, .degree.R; and PA1 S = specific gravity at 60.degree. F. PA1 K = the Watson-Nelson characterization factor of the hydrocarbon mixture, PA1 V = the absolute viscosity of said hydrocarbon mixture, PA1 S.sub.1 = the specific gravity at the temperature of the hydrocarbon mixture, PA1 S.sub.2 = the specific gravity of the hydrocarbon mixture adjusted to sixty degrees F., and PA1 A, b, and C are predetermined constants related to the temperature of said hydrocarbon mixture.
In general, the characterization factor is useful because it correlates with the average hydrocarbon type analysis, or quality of an oil. Thus, for example, the characterization factor can be used to distinguish a heavy oil of one crude from a similar oil of a different crude, a refined oil from the parent charge stock, and a higher VI (viscosity index) refined oil from a lower VI refined oil, both prepared from the same parent oil. Accordingly, it is extremely useful to continuously monitor the characterization factor of refinery process streams to distinguish among products, and to detect changes which occur during processing. For example, changes which can be detected include changes in crude oil quality or composition supplied to crude stills, changes in charge stock quality to refining or deasphalting processes, and changes in product oil quality from refining or deasphalting processes. Thus, the characterization factor can be used as the basis for control of various refinery operations.
Given the required data, i.e., the average boiling point and the specific gravity, it is relatively easy to calculate K in accordance with the above equation. However, the average boiling point is not a property that lends itself readily to continuous on-stream measurements. In fact, for very heavy oils, it is very difficult and sometimes impossible, to determine the average boiling point of the oil.
Consequently, and in accordance with the invention disclosed herein, I have provided an improved capability for the continuous on-stream determination of the Watson-Nelson characterization factor whereby various petroleum refining processes can be monitored or controlled automatically, or by computer, resulting in improved product quality and economy.